Hoop spring in a pressure reactive piston

ABSTRACT

Systems and methods are provided for varying a compression ratio in an engine having a pressure reactive piston. The pressure reactive piston may include a piston crown, and a spring positioned within the piston crown, wherein the spring includes a first ring, a second ring comprising a plurality of apertures, a rolling element positioned within each of the plurality of apertures, and a third ring. The first ring, the second ring, and the third ring of the spring may be arranged concentrically and the second ring may be positioned between the first ring and the third ring.

FIELD

The present disclosure relates to a spring positioned in a piston withinan internal combustion engine.

BACKGROUND AND SUMMARY

A pressure reactive piston (PRP) situated in a cylinder of an engine maycomprise a two-piece piston, wherein the two-piece piston includes apiston crown and a piston trunk. By integrating a separately actuatedpiston crown with a piston trunk, peak cylinder pressures may be reducedat higher loads without an additional control device. As such, theengine may operate at a higher compression ratio during lower loadconditions, and may operate at a lower compression ratio during higherload conditions. Thus, peak temperatures and pressures within thecombustion chamber may be reduced.

An example pressure reactive piston assembly is described by Brevick etal. in U.S. Pat. No. 5,755,192. Herein, the pressure reactive pistonassembly includes a trunk portion, a crown portion slidably mounted uponthe trunk portion, and a resilient element. The resilient elementextends between an interior surface of the crown portion and an uppersurface of the trunk portion and exerts a force to separate the crownportion from the trunk portion. In particular, the resilient elementconsists of four sets of Belleville springs.

The inventors herein have recognized potential issues with the approachidentified above. The Belleville spring sets included in the pressurereactive piston assembly may have a higher mass than desired. Further,the Belleville springs may have an uneven stress distribution. Furtherstill, Belleville springs may not deflect as desired during certainloads. Specifically, spring rates of Belleville springs may not besuitable for the desired application in a PRP. As a result, there may bean increased risk of knock and reduction in engine efficiency and fueleconomy.

The inventors herein have recognized the above issues and developed anapproach to at least partly address the above issues. In one exampleapproach, a system is provided including a piston crown, and a springpositioned within the piston crown, the spring including a first ring, asecond ring comprising a plurality of apertures, a rolling elementpositioned within each of the plurality of apertures, and a third ring,wherein the first ring, the second ring, and the third ring are arrangedconcentrically with the second ring positioned between the first ringand the third ring. In this way, a spring with lower mass may be usedwithin a pressure reactive piston.

For example, an engine may include a cylinder with a pressure reactivepiston. The pressure reactive piston may include two distinct pieces: atrunk portion and a crown portion, coupled to each other mechanically.Further, a hoop spring may be positioned within the crown portion of thepiston and may rest atop the trunk portion of the piston. The hoopspring may include a first ring, a second ring, and a third ringarranged in a concentric manner. The second ring may include a pluralityof apertures with each aperture including a rolling element. The firstring and the third ring may be made of steel, while the second ring maybe made of a polymer material. As such, the engine may function withvariable compression ratios as the crown portion of the piston slidesover the trunk portion of the piston and compresses the hoop spring.

In this way, a hoop spring housed within a two-piece pressure reactivepiston may provide variable compression ratio to the engine. By formingthe spring of three distinct lightweight elements, a mass of the hoopspring may be reduced. The reduced mass of the hoop spring can lower theweight of the two-piece pressure reactive piston, improving engineperformance. Moreover, the hoop spring may experience more uniformstress distribution, thereby increasing durability. Overall, engineefficiency may be improved while enhancing fuel economy.

The above advantages and other advantages, and features of the presentdescription will be readily apparent from the following DetailedDescription when taken alone or in connection with the accompanyingdrawings.

It should be understood that the summary above is provided to introducein simplified form a selection of concepts that are further described inthe detailed description. It is not meant to identify key or essentialfeatures of the claimed subject matter, the scope of which is defineduniquely by the claims that follow the detailed description.Furthermore, the claimed subject matter is not limited toimplementations that solve any disadvantages noted above or in any partof this disclosure. Finally, the above explanation does not admit any ofthe information or problems were well known.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic diagram of an example engine.

FIG. 2 depicts an exploded view of an example pressure reactive pistonin an engine, such as the engine of FIG. 1.

FIG. 3 illustrates an exploded view of an example hoop spring in theexample pressure reactive piston of FIG. 2.

FIG. 4 is perspective view of the example hoop spring when fullyassembled.

FIGS. 5A, 5B, 5C, and 5D, show schematic, cross-sectional views ofalternative embodiments of the example hoop spring in the examplereactive piston of FIG. 2.

FIGS. 6A and 6B portray a schematic, cross-sectional view of thepressure reactive piston in a compressed state (FIG. 6A) and in anexpanded state (FIG. 6B).

FIG. 7 is an example flow chart showing adjustments to spark timingbased on an estimated compression ratio of the engine.

DETAILED DESCRIPTION

The following description relates to systems and methods for varying acompression ratio of an engine, such as the example engine depicted inFIG. 1. The compression ratio may be adjusted via including a springassembly, or a hoop spring, within a two-piece piston, the two-piecepiston having a trunk portion and a crown portion, as shown in FIG. 2.In particular, the hoop spring may include a first ring, a second ring,and a third ring, wherein the second ring may include a pluraltiy ofrolling elements (FIG. 3). As such, the hoop spring may assembledtogether as shown in FIG. 4, and mounted on the trunk portion of thepiston. Features of the hoop spring may be varied depending on desiredspring rate and preload characteristics (FIGS. 5A-5D). In oneembodiment, the hoop spring, may control the compression ratio by movingbetween a first state, or compressed position (FIG. 6A), and a secondstate, or expanded position (FIG. 6B). Further, spark timing may beadjusted based on the compression ratio of the engine, wherein thecompression ratio is based on an age of the hoop spring within eachcylinder of the engine, as shown in an example method of FIG. 6.

It will be noted that though the following description discusses the useof the hoop spring in a pressure reactive piston within a cylinder of anengine, the hoop spring may also be utilized in pumps and/or motorswithout departing from the scope of this disclosure.

Referring now to FIG. 1, a schematic diagram showing one cylinder ofmulti-cylinder engine 10, which may be included in a propulsion systemof an automobile, is illustrated. Engine 10 may be controlled at leastpartially by a control system including controller 12 and by input froma vehicle operator 132 via an input device 130. In this example, inputdevice 130 includes an accelerator pedal and a pedal position sensor 134for generating a proportional pedal position signal PP.

Combustion chamber 30 (also termed, cylinder 30) of engine 10 mayinclude combustion chamber walls 32 with piston 36 positioned therein.Piston 36 may be a pressure reactive piston comprising two sections: acrown and a trunk (not shown). A spring may be positioned within thepiston to provide variation in compression ratio of engine 10, as willbe described further in reference to FIGS. 2-5. Piston 36 may be coupledto crankshaft 40 via a connecting rod 38 so that reciprocating motion ofthe piston is translated into rotational motion of the crankshaft.Crankshaft 40 may be coupled to at least one drive wheel of a vehiclevia an intermediate transmission system. Further, a starter motor may becoupled to crankshaft 40 via a flywheel to enable a starting operationof engine 10.

In this example, intake valve 52 and exhaust valves 54 may be controlledby cam actuation via respective cam actuation systems 51 and 53. Camactuation systems 51 and 53 may each include one or more cams and mayutilize one or more of cam profile switching (CPS), variable cam timing(VCT), variable valve timing (VVT), and/or variable valve lift (VVL)systems that may be operated by controller 12 to vary valve operation.The position of intake valve 52 and exhaust valve 54 may be determinedby position sensors 55 and 57, respectively. In alternative embodiments,intake valve 52 and/or exhaust valve 54 may be controlled by electricvalve actuation. For example, cylinder 30 may alternatively include anintake valve controlled via electric valve actuation and an exhaustvalve controlled via cam actuation including CPS and/or VCT systems.

Fuel injector 66 is shown coupled directly to combustion chamber 30 forinjecting fuel directly therein in proportion to the pulse width ofsignal FPW received from controller 12 via electronic driver 99. In thismanner, fuel injector 66 provides what is known as direct injection offuel into combustion chamber 30. The fuel injector may be mounted in theside of the combustion chamber or in the top of the combustion chamber,for example. Fuel may be delivered to fuel injector 66 by a fuel system(not shown) including a fuel tank, a fuel pump, and a fuel rail. In someembodiments, combustion chamber 30 may alternatively or additionallyinclude a fuel injector arranged in intake manifold 44 in aconfiguration that provides what is known as port injection of fuel intothe intake port upstream of combustion chamber 30.

Continuing with FIG. 1, intake passage 42 may include a throttle 62having a throttle plate 64. In this particular example, the position ofthrottle plate 64 may be varied by controller 12 via a signal providedto an electric motor or actuator included with throttle 62, aconfiguration that is commonly referred to as electronic throttlecontrol (ETC). In this manner, throttle 62 may be operated to vary theintake air provided to combustion chamber 30 among other enginecylinders. The position of throttle plate 64 may be provided tocontroller 12 by throttle position signal TP. Intake passage 42 mayinclude a mass air flow sensor 120 and a manifold air pressure sensor122 for providing respective signals MAF and MAP to controller 12.Ignition system 88 can provide an ignition spark to combustion chamber30 via spark plug 92 in response to spark advance signal SA fromcontroller 12, under select operating modes. Though spark ignitioncomponents are shown, in some embodiments, combustion chamber 30 or oneor more other combustion chambers of engine 10 may be operated in acompression ignition mode, with or without an ignition spark. As such,in the compression ignition mode, homogeneous charge compressionignition (HCCI) or stratified charge combustion may be applied. Further,engine 10 may be a two-stroke engine in one example. However, engine 10depicted in FIG. 1 may be a four-stroke engine.

Engine 10 may further include one or more pressure sensors (not shown)for sensing pressure in combustion chamber 30, and/or abnormalcombustion events and differentiating abnormal combustion events due toknocking from those indicative of pre-ignition. For example, input froman in-cylinder pressure sensor may be used to estimate a compressionratio in the cylinder. As such, the pressure sensor may be anin-cylinder pressure transducer.

Exhaust gas sensor 126 (e.g., exhaust oxygen sensor) is shown coupled toexhaust passage 58 upstream of emission control device 70. Sensor 126may be any suitable sensor for providing an indication of exhaust gasair/fuel ratio such as a linear oxygen sensor or UEGO (universal orwide-range exhaust gas oxygen), a two-state oxygen sensor or EGO, a HEGO(heated EGO), a NOx, HC, or CO sensor. In another embodiment, the enginemay include an additional exhaust gas sensor so that the engine includestwo exhaust gas sensors, both positioned upstream of the emissioncontrol device 70 (e.g., upstream of any and all catalysts in the enginesystem). For example, the exhaust gas sensor 126 may be the air/fuelratio sensor while the second exhaust gas sensor may be an exhaustsensor dedicated for determining engine exhaust pressure and not fordetermining air/fuel ratio.

Emission control device 70 is shown arranged along exhaust passage 58downstream of exhaust gas sensor 126. Device 70 may be a three waycatalyst (TWC), NOx trap, various other emission control devices, orcombinations thereof. In some embodiments, during operation of engine10, emission control device 70 may be periodically reset by operating atleast one cylinder of the engine within a particular air/fuel ratio.

Further, though now shown, an exhaust gas recirculation (EGR) system mayroute a desired portion of exhaust gas from exhaust passage 58 to intakemanifold 44 via an EGR passage. The amount of EGR provided to intakemanifold 44 may be varied by controller 12 via an EGR valve. Under someconditions, the EGR system may be used to regulate the temperature ofthe air and fuel mixture within the combustion chamber, thus providing amethod of controlling the timing of ignition during some combustionmodes.

Controller 12 is shown in FIG. 1 as a microcomputer, includingmicroprocessor 102, input/output ports 104, an electronic storage mediumfor executable programs and calibration values shown as read only memorychip 106 in this particular example, random access memory 108, keepalive memory 110, and a data bus. Controller 12 may receive varioussignals from sensors coupled to engine 10, in addition to those signalspreviously discussed, including measurement of inducted mass air flow(MAF) from mass air flow sensor 120; engine coolant temperature (ECT)from temperature sensor 112 coupled to cooling sleeve 114; a profileignition pickup signal (PIP) from Hall effect sensor 118 (or other type)coupled to crankshaft 40; throttle position (TP) from a throttleposition sensor; and absolute manifold pressure signal, MAP, from sensor122. Engine speed signal, RPM, may be generated by controller 12 fromsignal PIP. Further, controller 12 may estimate a compression ratio ofthe engine based on measurements from a pressure transducer positionedin the cylinder 30 (not shown).

The controller 12 receives signals from the various sensors of FIG. 1and employs the various actuators of FIG. 1 to adjust engine operationbased on the received signals and instructions stored on a memory of thecontroller.

Storage medium read-only memory 106 can be programmed with computerreadable data representing instructions executable by processor 102 forperforming the methods described below as well as other variants thatare anticipated but not specifically listed.

As described above, FIG. 1 shows only one cylinder of a multi-cylinderengine, and each cylinder may similarly include its own piston, set ofintake/exhaust valves, fuel injector, spark plug, etc.

FIG. 2 illustrates an exploded view of a piston, such as piston 36 ofFIG. 1, wherein the piston may be reciprocably mounted within an enginecylinder, such as cylinder 30 of engine 10, as shown in FIG. 1. FIG. 2is drawn to scale, although other relative dimensions may be used. Asmentioned earlier, piston 36 may be a pressure reactive piston (PRP)that reacts to pressure variations within its corresponding cylinder.Piston 36 may be a two-piece piston comprising a crown portion 202 (alsotermed piston crown 202) and a trunk portion 250 (also termed, pistontrunk 250). Piston 36 may be coupled to a connecting rod (e.g.connecting rod 38 of FIG. 1) via piston trunk 250. As such, piston trunk250 may be coupled (e.g. mechanically) to piston crown 202.

Further, a hoop spring 220 (also termed, spring 220) may rest atop thetrunk portion 250. Specifically, spring 220 may rest on an upper portionof piston trunk 250, wherein the upper portion is closer to the pistoncrown 202 and away from connecting rod 38. Further still, hoop spring220 may be fully enclosed within the crown portion 202. As such, each ofthe crown portion 202, the trunk portion 250, and the hoop spring 220may have a common central axis 280. Alternatively, a central axis of thecrown portion 202, a central axis of the trunk portion 250, and acentral axis of the spring 220 may be parallel to each other. In theexample of FIG. 2, the central axis of the crown portion 202, thecentral axis of the trunk portion 250, and the central axis of thespring 220 may be the same (e.g., central axis 280).

In the depicted embodiment, crown portion 202 comprises a cylindricalinterior wall surface 206, a cylindrical exterior wall surface 204, aroof surface 262, and a lower rim 264, as shown in FIG. 2. Interior wallsurface 206 and exterior wall surface 204 of crown portion 202 may eachbe parallel to each other and to a central axis 280. Roof surface 262may be adjacent to and orthogonal relative to each of the interior wallsurface 206 and exterior wall surface 204.

In addition, crown portion 202 may include a plurality of piston rings(not shown) mounted within a plurality of piston ring grooves 260,wherein the piston ring grooves 260 are arranged circumferentially alongthe exterior wall surface 204 of crown portion 202. In one example,there may be two piston ring grooves 260. In another example, there maybe three piston ring grooves 260. In yet another example, piston 36 mayinclude additional or fewer piston ring grooves. Under some conditions,mounting the plurality of piston rings circumferentially around crownportion 202 via the plurality of piston ring grooves 260 may promoteefficient heat transfer from the piston to the cylinder wall and enabledynamic system damping. In one embodiment, crown portion 202 of piston36 may be made from steel. In other embodiments, crown portion 202 maybe made from another durable material able to withstand highertemperatures and resist deformation caused by thermal stress duringengine cycles.

Piston 36 may be mechanically coupled to connecting rod 38 (shown inFIG. 1) via wrist pin 254, which is housed in wrist pin bore 256 oftrunk portion 250 of piston 36. In the depicted embodiment, wrist pin254 of piston 36 may be shorter in length than a wrist pin in anon-pressure reactive piston. For example, wrist pin 254 may be indentedtoward a center of the trunk portion 250 relative to an outercircumference of crown portion 202. The shortened length of wrist pin254 may allow the crown portion 202 to slide over trunk portion 250unimpeded by wrist pin 254.

Crown portion 202 of piston 36 may be slidably mounted upon trunkportion 250. Thus, crown portion 202 of piston 36 may slide over an edgeof trunk portion 250. It will be appreciated that while crown portion202 may slide and shift its position, the trunk portion 250 of piston 36may not be capable of moving. As such, trunk portion 250 may besubstantially fixed relative to the crown portion 202. The trunk portion250 may include an upper rim 258 adjacent to a protuberance 252. In someexamples, the lower rim 264 of the crown portion 202 may abut against,and directly contact, the upper rim 258 of the trunk portion 250, asshown below in FIG. 5A during one or more engine operating conditions,such as higher engine loads. In other examples, during lower engineloads, the lower rim 264 of the crown portion 202 may not abut againstand may not directly contact the upper rim 258 of the trunk portion 250,as shown below in FIG. 5B.

Hoop spring 220 may be arranged atop the trunk portion 250.Specifically, hoop spring 220 may be positioned concentrically aroundprotuberance 252 of trunk portion 250. Further, hoop spring 220 may besurrounded by the crown portion 202. As shown in the explodedperspective view of FIG. 2 and in FIG. 3, the hoop spring 220 comprisesan outer first ring 222 (also termed, first ring 222), an intermediatesecond ring 224 (also termed, second ring 224), and an inner third ring226 (also termed, third ring 226). Each of the first ring 222, secondring 224, and third ring 226 in the hoop spring 220 may be substantiallycylindrical in shape. The first ring 222, second ring 224, and thirdring 226 may be arranged in a concentric manner. To elaborate, a centralaxis of the first ring may be parallel to a central axis of the secondring. Further, the central axis of the second ring may be parallel to acentral axis of the third ring. In the depicted example, the centralaxis of the first ring may be the same as the central axis of the secondring. Further still, the central axis of the second ring may be the sameas the central axis of the third ring. Additionally, the central axes ofeach of the first ring, the second ring, and the third ring may besubstantially parallel to, and the same as, central axis 280. Saidanother way, the central axis of the first ring coincides with each ofthe central axis of the second ring and the central axis of the thirdring. Further, the central axes of the crown portion 202 and trunkportion 250 may each coincide with each of the central axis of the firstring 222, the central axis of the second ring 224, and the central axisof the third ring 226.

Thus, the third ring 226 may be nested in the second ring 224, and thesecond ring 224 may be nested in the first ring 222, as shown in FIG. 4.As such, the second ring may be positioned between the first ring andthe third ring. In this way, the first ring 222 may be in direct contactwith the second ring 224, and may not be in direct contact with thethird ring 226, and the third ring 226 may be in direct contact with thesecond ring 224 but may not be in direct contact with the first ring222.

Further still, each of the first ring 222, second ring 224, and thirdring 226 in the hoop spring 220 may be tapered at an end of each ringproximal the trunk portion 250. To elaborate, each of the first ring222, second ring 224, and third ring 226 may be tapered at an end thatis away from (e.g., distal) crown portion 202. For example, an innerlateral surface 232 and outer lateral surface 242 of the first ring 222may be angled towards the central axis of the first ring 222 (which maybe the same as central axis 280). Similarly, an inner lateral surface234 and outer lateral surface 244 of the second ring 224 may be angledtowards the central axis of the second ring 224 which may be the same ascentral axis 280. Likewise, an inner lateral surface 236 and outerlateral surface 246 of the third ring 226 may be angled towards thecentral axis of the third ring 226 which may be the same as central axis280.

It will be appreciated that each of the rings of hoop spring 220 may betapered by the same amount. In other words, each of the inner lateralsurfaces and each of the outer later surfaces of each of the rings maybe angled substantially similarly relative to central axis 280 allowingthe third ring to be surrounded by the second ring, and for the firstring to encompass the second ring.

In an alternative embodiment, each of the first ring 222, second ring224, and third ring 226 may be tapered, or angled, toward the centralaxis 280 at an end proximal crown portion 202 (and away from trunkportion 250).

In one example, the inner lateral surface 232 and outer lateral surface242 of the first ring 222 may be angled 5 degrees relative to thecentral axis 280 at an end proximal the trunk portion 250. In anotherexample, inner lateral surface 234 and outer lateral surface 244 of thesecond ring 224 may be similarly angled 5 degrees relative to thecentral axis 280 at an end proximal the trunk portion 250. In yetanother example, inner lateral surface 236 and outer lateral surface 246of the third ring 226 may be angled 5 degrees relative to the centralaxis 280 at an end proximal the trunk portion 250. In one embodiment,each of the inner lateral surface and outer later surface of each of thefirst ring 222, second ring 224, and third ring 226 may be angled 10degrees relative to the central axis 280 at an end proximal trunkportion 250. In another one embodiment, each of the inner lateralsurface and outer later surface of each of the first ring 222, secondring 224, and third ring 226 may be angled at any degree between 5 and10 degrees relative to the central axis 280 at an end proximal trunkportion 250. Alternatively, each of the inner lateral surface and outerlateral surface of each of the first ring 222, second ring 224, andthird ring 226 may be angled as described above relative to the centralaxis 280 at an end proximal crown portion 202.

As shown in FIG. 2 (and in FIG. 3), a circumference of the second ring224 comprises one or more apertures 228, wherein each of the apertures228 retains a rolling element 230. On the other hand, a circumference ofthe first ring 222 and a circumference of the third ring 226 do notinclude a plurality of apertures 228 and/or rolling elements 230. In oneexample, the rolling element 230 may be a ball. As such, the rollingelements may be spheres formed with precise design parameters. Forexample, each of the rolling elements 230 may be 2 millimeters indiameter. In another example, the diameter of each rolling element maybe 3 millimeters. In another example, the rolling element 230 may be acylindrical rolling element. The size of the apertures 228 may be basedon the size of the rolling elements. In one example, each of the rollingelements 230 may be made of steel. In another example, the rollingelements may be formed from plastic. In yet another example, materialused to form the rolling elements may be a ceramic.

It will be noted that the circumference of the second ring may include aplurality of apertures, each of the plurality of apertures accommodatinga single rolling element. In one example, the second ring may include100 apertures. In another example, the number of apertures, andtherefore, number of rolling elements, may be 150. In yet anotherexample, the hoop spring may include 200 rolling elements situated in200 apertures on the second ring. In one embodiment, the first ring 222and third ring 226 may each be made of steel, while the second ring 224may be composed of a polymer, such as nylon. Further, the first ring 222and the third ring 226 may be hardened by heat treatment. In anotherembodiment, the first ring 222 and third ring 226 may each be made ofanother metal, such as aluminum, titanium, and/or a metal alloy such asbronze. Alternatively, the hoop spring may be formed of a compositematerial. Combinations of the above may also be possible. For example,the third cylinder may be made of aluminum while the first cylinder ismade of steel. In another example, the third cylinder may be made ofbronze while the first cylinder is made of aluminum.

As shown in FIG. 2, the outer lateral surface 246 (also termed, outercircumference region 246) of the third ring 226 may include a pluralityof axial grooves 296. Likewise, the inner lateral surface 232 (alsotermed, inner circumference region 232) of the first ring 222 maycomprise a plurality of axial grooves 292. Each axial groove may be inface-sharing contact with a corresponding rolling element 230 on thesecond ring 224. As such, each rolling element 230 may be inface-sharing contact with two axial grooves: a first axial groove onouter lateral surface 246 of third ring 226 and a second axial groove oninner lateral surface of first ring 222. Further, each rolling element230 may be in face-sharing contact with the two axial grooves at thesame time.

Each rolling element 230 retained in apertures of the second ring 224may slide along each of a corresponding first axial groove and acorresponding second axial groove. Each first axial groove (e.g., 296)and second axial groove (e.g., 292) may be complementary but may not bein face-sharing contact with each other when the hoop spring 220 isassembled. Each axial groove may allow controlled rolling of the secondring 222 against the inner lateral surface 232 of the first ring 222 andthe outer lateral surface 246 of the third ring 226 by enabling linecontact and/or less unit loading. As such, the number of axial grooves296 on the outer circumference region 246 of third ring 226 and numberof the axial grooves 292 on the inner circumference region 232 of thefirst ring 222 may be the same as the total number of rolling elements230 of the second ring 224.

The depth of each of the axial grooves 292 and axial grooves 296 may bebased on the diameter of the rolling elements 230. For example, if therolling elements are each 2 mm in diameter, each of the axial grooves292 and each of axial grooves 296 may be 1 mm deep into the first ring222 and the third ring 226, respectively. Further, in another example,each of axial grooves 292 may be the same length as each of axialgrooves 296 along central axis 280. In one example, each axial groove292 and axial groove 296 may be 3 mm in length along central axis 280.In another example, each axial groove 292 and axial groove 296 may be 4mm in length along central axis 280.

In another embodiment, axial grooves 292 on the inner lateral surface232 of the first ring 222 and axial grooves 296 on the outer lateralsurface 246 of the third ring 226 may not be provided.

An alternative embodiment of a hoop spring may include cylindricalrolling elements instead of spherical rolling elements, as describedabove. In this alternative embodiment a flat indentation (not shown inFigures) may be provided for each cylindrical rolling element on thesecond ring 224. Further, a plurality of flat indentations may be formedon the inner lateral surface 232 of the first ring 222 and/or on theouter lateral surface 246 of the third ring 226. The number of flatindentations on the inner lateral surface 232 of the first ring 222 maybe the same as a number of cylindrical rolling elements on the secondring 224. Similarly, the number of flat indentations on the outerlateral surface 246 of the third ring 226 may be the same as a number ofcylindrical rolling elements on the second ring 224.

Each of the flat indentations may allow controlled rolling of the secondring 224 against the inner lateral surface 232 of the first ring 222 andthe outer lateral surface 246 of the third ring 226 by enabling linecontact and/or lesser unit loading. In one example, each cylindricalelement retained in the second ring 224 may slide along itscomplementary and face-sharing flat indentation on each of the thirdring 226 and first ring 222 when the hoop spring 220 is assembled.

It will be appreciated that various dimensions of the components of thehoop spring 220 may be varied without departing from the scope of thisdisclosure. Further, the coefficient of friction may be varied to enablesmoother movement of the elements of the hoop spring.

It will also be noted that hoop spring 220 may be preloaded. The preloadon the hoop spring may be such that deflection or displacement of thespring via compression may occur only at loads higher than the preload.The preload on the hoop spring 220 may reduce relative movement betweenthe upper crown portion 202 and lower trunk portion 250 of piston 36until cylinder pressure exceeds the preload on the hoop spring. In oneexample, the preload on hoop spring 220 may be approximately 2000 lbs(or 8.9 kN). In another example, the preload on hoop spring 220 may be2500 lbs (or 11 kN).

In one embodiment, hoop spring 220 may be axially loaded by crownportion 202 when cylinder pressure exceeds the preload on the hoopspring 220. Herein, the third ring 226 may slide against and past theplurality of rolling elements 230 retained in the apertures 228 of thesecond ring 224. As such, the third ring 226 may move towards the secondring 224. Specifically, the outer lateral surface 246 of the third ring226 may slide on the inner lateral surface 234 of second ring 224.Concomitantly, the second ring 224 may slide against the inner lateralsurface 232 of the first ring 222 enabled by the plurality of rollingelements 230 of second ring 224. As such, each of the second ring 224and third ring 226 may slide in a first direction. In one example, thefirst direction is a downward direction towards the trunk portion 250and the wrist pin 254.

Thus, when cylinder pressure exceeds the preload on the hoop spring 220,the crown portion 202 of piston 36 slides towards trunk portion 250. Toelaborate, crown portion 202 exerts a force on upper rim 238 of the hoopspring 220 that exceeds the preload on spring 220 causing compression ofthe hoop spring. The compression of the hoop spring may be referred toas the spring being in a compressed position or state. Compression ofthe hoop spring may be a function of a spring rate (or spring constant)of the spring.

As such, the third ring 226 may be pressed into each of the second ring224 and first ring 222, the third ring 226 sliding on the rollingelements 230 positioned in the plurality of apertures 228 in the secondring 224 as a distance between the crown of the piston and the pistontrunk decreases. In this way, the third ring 226 (also termed, thirdannular element), may be fitted more tightly within the second ring 224(also termed, second annular element). Further, the first ring 222 (alsotermed, first annular element), may enclose second ring 224 more snugly.In this compressed state, the third ring 226 may experience compressionwhile the first ring 222 may experience tension.

When the cylinder pressure reduces to below the preload of the hoopspring 220, the third ring 226 may slide against and past the pluralityof rolling elements 230 retained in apertures 228 of second ring 224 andmay expand away from the second ring 224. Specifically, outer lateralsurface 246 of third ring 226 may slide along the inner lateral surface234 of second ring 224. Concomitantly, the plurality of rolling elements230 of second ring 224 may slide against the inner lateral surface 236of the first ring 222, each of the second ring 224 and third ring 226sliding in a second direction. In one example, the second direction isan upward direction away from trunk portion 250 towards crown portion202 of the piston 36. Further, each of the first ring, the second ring,and the third ring may resume their positions relative to each other andthe crown portion 202.

As such, when cylinder pressure is lower than the preload on hoop spring220, the crown portion 202 of piston 36 may not exert a force on theupper rim 238 of the hoop spring 220. Herein, the hoop spring may be ina released position (also termed, expanded state). In this releasedposition, the third ring 226 may be fitted less forcefully within thesecond ring 224 relative to when hoop spring 220 is in the compressedposition. Further, the first ring 222 may enclose each of the secondring 224 and third ring 226 in a less constricted manner relative tothat in the compressed position of the hoop spring.

A retainer ring 212 may be provided to couple the crown portion 202 tothe trunk portion 250. The retainer ring 212 may have a thread, or aninterior ledge 214, to which crown portion 202 may be mechanicallycoupled. As such, the retainer ring 212 may allow stable assembly of thecrown portion 202 to the trunk portion 250, thereby enabling the desiredpreload of hoop spring 220. In an example, the retainer ring 212 may bea split ring. The retainer ring 212 may be cylindrical in shape and mayhave a distinct width relative to the widths of each of the first ring,the second ring, and the third ring of the hoop spring. As such, theretainer ring 212 may not be tapered. In the depicted embodiment, theretainer ring 212 may not couple to and may not directly contact thehoop spring 220, as to not impede or disrupt the sliding of the hoopspring 220 in the first and/or second direction. The depicted retainerring 212 may be threaded to enable assembly of the piston crown 202 tothe piston trunk 250. In an alternative embodiment, the retainer ringmay be furnace brazed to the piston crown after assembly.

In another embodiment, hoop spring 220 may rest directly upon upper rim258 of the trunk portion 250. Herein the retainer ring 212 maycircumferentially surround and enclose the third ring 226 around itsouter lateral surface 242 in order to reduce lateral motion of hoopspring 220. In yet another embodiment, a retainer ring may not beprovided. Additional components may be used to arrange the hoop spring220 within the crown portion 202 and/or on top of the trunk portion 250without departing from the scope of this disclosure. Further, the crownportion 202 of piston 36 may be coupled to the trunk portion 250 ofpiston 36 via one or more methods known in the art to achieve stabilityto the piston 36.

In this way, a system may be provided, comprising a piston crown, and aspring positioned within the piston crown, the spring including a firstring, a second ring comprising a plurality of apertures, a rollingelement positioned with each of the plurality of apertures, and a thirdring, wherein the first ring, the second ring, and the third ring arearranged concentrically with the second ring positioned between thefirst ring and the third ring. In addition, a piston trunk may becoupled to the piston crown, and the spring may be positioned on a topof the piston trunk. As such, the piston trunk and the piston crown forma piston of a cylinder within an engine, and the piston trunk may becoupled to a connecting rod. By providing the spring between the pistoncrown and piston trunk, the piston crown may be movable relative to thepiston trunk, while piston trunk may be fixed relative to the pistoncrown.

The first ring may be in direct contact with the second ring, and maynot be in direct contact with the third ring and wherein, the third ringmay be in direct contact with the second ring but may not be in directcontact with the first ring. Further, a central axis of the first ringmay be parallel to each of a central axis of the second ring and acentral axis of the third ring. Similarly, a central axis of the pistoncrown may coincide with each of the central axis of the first ring, thecentral axis of the second ring, and the central axis of the third ring.The first ring, the second ring, and the third ring may be shaped astapered cylinders. In one embodiment, the first ring and the third ringmay be made of steel, and the second ring may be made of a polymer.

Turning now to FIG. 3, it is an exploded view of hoop spring 220illustrating each of the first ring 222, second ring 224, and third ring226 of the hoop spring, as described above in reference to FIG. 2. FIG.3 also includes a magnified view 300, indicated by a circled dash-dotline, of the plurality of apertures 228 on the circumference of secondring 224. As shown in magnified view 300, each aperture 228 retainsrolling element 230. In one embodiment, each of the rolling elements 230may be substantially spherical in shape. Further, each of the rollingelements may rotate about a single axis. In one example, the single axismay be substantially orthogonal, or perpendicular to, the central axis280. As such, substantially all the rolling elements 230 may rotateabout the same single axis.

Each rolling element 230 may have a diameter substantially similar to athickness, denoted herein as T1, of the second ring 224. For example,each rolling element 230 in the plurality of apertures 228 of secondring 224 may directly contact the inner lateral surface 232 of the firstring 222. At the same time, each rolling element 230 may also be in facesharing (or direct) contact with the outer lateral surface 246 of thethird ring 226. In this way, the rolling elements 230 may allow thesecond ring 224 to slide past the inner lateral surface 232 of the firstring 222. Similarly, the outer lateral surface 246 of the third ring 226may slide against the rolling elements 230 in the second ring 224.

In one example, e.g., when spring 220 is compressed, each of the secondring 224 and third ring 226 may slide in the first direction (e.g.,downward) into the first ring 222 toward trunk portion 250, as will bedescribed below in reference to FIG. 5A. In another example, e.g., whenspring 220 expands, each of the second ring 224 and third ring 226 mayslide in the second direction (e.g., upward) from the first ring 222toward piston crown 202, as will be described below in reference to FIG.5B. As such, the hoop spring 220 may be compressed (and may also bereleased) in an axial direction parallel to central axis 280. Further,axial travel of the entire hoop spring when under compression may be aspecifically designed parameter. In one example, the hoop spring may bedesigned to have an axial travel distance of 3 mm. In another example,the axial travel for the hoop spring may be 4 mm. In yet anotherexample, the axial travel for the hoop spring may be a distance between3 and 4 mm.

Thus, the hoop spring 220 described in FIGS. 2-3 may enable a methodcomprising, responsive to pressure in a cylinder of an engine exceedinga threshold, displacing a crown of a piston toward a trunk of thepiston, the piston reciprocating within the cylinder, and compressing athird ring of a spring into each of a second ring and a first ring ofthe spring, the third ring sliding past a plurality of rolling elementsincluded within a plurality of apertures in the second ring of thespring, wherein the second ring is positioned between the first ring andthe third ring. The threshold may be a preload on the spring positionedin the piston. The pressure in the cylinder of the engine may exceed thethreshold during a power stroke in the cylinder. Specifically, thepressure in the cylinder of the engine may exceed the threshold duringthe power stroke of the engine in a range between 10 to 20 degrees afterthe piston reaches top-dead-center (TDC) position in the correspondingcylinder. In one example, pressure in the cylinder may exceed thepreload of the hoop spring approximately 10 degrees after TDC positionof the corresponding piston in the power stroke. In another example,cylinder pressure may increase to higher than the threshold at about 20degrees after TDC position of the piston in the power stroke. The methodmay further include responsive to pressure in the cylinder of the enginedecreasing below the threshold, releasing the third ring of the springfrom each of the second ring and first ring of the spring, anddisplacing the crown of the piston away from the trunk of the piston.The pressure in the cylinder may decrease below the threshold during anexhaust stroke in the cylinder subsequent to the power stroke.

In one embodiment, the third ring may be nested within the second ring,and the second ring may be nested within the first ring, such that acentral axis of the first ring is aligned with each of a central axis ofthe second ring and a central axis of the third ring. Further, each ofthe first ring, second ring, and third ring may be shaped as taperedcylinders. The spring may be substantially enclosed within the crown,and the spring may be positioned on top of the trunk of the piston.

FIG. 4 illustrates a perspective view of the hoop spring 220 as observedfrom a top of the hoop spring. The hoop spring, herein also referred toas a spring assembly, includes the outer first ring 222, intermediatesecond ring 224 (shown here as a dotted surface), and inner third ring226. FIG. 4 illustrates the first ring 222, the second ring 224, and thethird ring 226 assembled together to form hoop spring 220. Specifically,the hoop spring 220 may be placed, as assembled, within piston 36.

In one embodiment, third ring 226 may have a smaller diameter than thesecond ring 224, and the second ring 224 may have a smaller diameterthan the first ring 222. As such, the third ring 226 may have a smallerdiameter relative to the diameters of each of first ring 222 and secondring 224. To elaborate, an outer diameter and an inner diameter of thethird ring may be smaller than an outer diameter and an inner diameterof the first ring, respectively. Accordingly, the third ring 226 may benested in the second ring 224, and the second ring 224, having the thirdring 226 concentrically mounted therein, may be nested in the first ring222. Thus, each of the first ring 222, second ring 224, and third ring226 may share the same central axis 280 (as shown in FIGS. 2 and 3).

It will be appreciated that the diameters of each ring of the hoopspring may be varied. As described earlier, each of the rings of thehoop spring may be tapered at one end. For example, each of the firstring, the second ring, and the third ring may be angled at 5 degrees perside (or a total of 10 degrees) relative to the central axis 280. Inanother example, each of the first ring, the second ring, and the thirdring may be tapered at 7 degrees per side relative to the central axis280. As discussed above in reference to FIG. 2, each of the innerlateral surface and outer lateral surface of each of the first ring 222,second ring 224, and third ring 226 may be angled relative to thecentral axis 280 at an end proximal trunk portion 250. In alternativeembodiments, each of the inner lateral surface and outer lateral surfaceof each of the first ring 222, second ring 224, and third ring 226 maybe angled relative to the central axis 280 at an end proximal crownportion 202. Thus, the end of each ring of the hoop spring closer to thepiston crown may be tapered. Further still, a height of each ring of thehoop spring may be substantially similar. The height of each ring of thehoop spring may depend on the height of the crown portion 202 of thepiston 36. In one example, each ring of the hoop spring may be 15 mm inheight. In another example, each ring of the hoop spring may be 20 mm inheight. It will be noted that the heights of each ring may be differentthan the examples provided above without departing from the scope ofthis disclosure. Likewise, a thickness of each ring of the hoop springmay be substantially similar. The dimensions of the rings of the hoopspring may also affect the weight (or mass) of the hoop spring. As such,the weight (or mass) of the hoop spring 220 may be reduced by selectingspecific dimensions of the hoop spring. Further still, by using apolymer material for at least the second ring of the hoop spring, theweight (or mass) of the hoop spring 220 may be further diminished.

Turning now to FIGS. 5A-5D, cross-sectional views of alternativeembodiments of the hoop spring are depicted. The hoop springs in FIGS.5A-5D may be different arrangements of an example hoop spring, such ashoop spring 220. Further, the cross-sectional views of the hoop springsare schematic. As such, designing and forming a hoop spring may comprisemodifying one or more of multiple variables including material,coefficient of friction, a profile of the hoop spring, as well as across section of the hoop spring. Each of FIGS. 5A, 5B, 5C, and 5Ddepict two positions of the depicted hoop spring: a compressed state anda more relaxed state. The compresses state of the example hoop springsmay occur when cylinder pressure exceeds a preload of the example hoopspring of each of these embodiments. As explained earlier, in responseto cylinder pressure being greater than the preload of the hoop spring,the piston crown may move towards the piston trunk causing compressionof the hoop spring located within the piston crown. When cylinderpressure is lower than the preload of the hoop spring (e.g., in anexhaust stroke subsequent to a prior power stroke), the distinct ringsof the hoop spring may expand, and release, away from each other. Assuch, the hoop spring may be in the expanded state (or relaxed state) inresponse to the cylinder pressure being lower than the preload of thehoop spring.

In FIG. 5A, an example hoop spring having a nested configuration isdepicted. Herein, the example hoop spring includes five rings: firstring 502, second ring 504, third ring 505, fourth ring 506, and fifthring 508. Each of the five rings may be nested together and positionedconcentrically (in relation to central axis 501) as shown. Specifically,first ring 502 is positioned at the exterior (e.g., outermost of thefive rings) of the embodiment, and substantially surrounds the remainingfour rings. The fifth ring 508 is the innermost ring of the five ringsand is substantially surrounded by fourth ring 506. Further, third ring505 may be positioned in the middle substantially surrounded by secondring 504. Further still, third ring 505 substantially encloses fourthring 506.

As depicted, each of second ring 504 and fourth ring 506 include aplurality of apertures, each aperture 509 including a rolling element507. Thus, both second ring 504 and fourth ring 506 have a plurality ofrolling elements 507 included along their respective circumferences.Further, each of first ring 502, third ring 505, and fifth ring 508 maynot include any apertures on their circumference. Further still, none ofthe first ring 502, third ring 505, and fifth ring 508 include anyrolling elements. As such, the spring embodiment depicted in FIG. 5Aincludes two rings with rolling elements and three rings without rollingelements.

In one example, the third ring 505 may be cylindrical in shape, and mayhave a diameter between each of the diameter of the first ring 502 andfifth ring 508. In other words, the diameter of third ring 505 may begreater than that of fifth ring 508 and at the same time, the diameterof the third ring 505 may be smaller than the diameter of the first ring502. In this example, the fourth ring 506 may have a diameter betweeneach of the diameter of the third ring 505 and the diameter of the fifthring 508.

First image 510 of FIG. 5A shows the example hoop spring in a compressedstate. Herein, a downward force (e.g., cylinder pressure) may be exertedon the hoop spring in the first direction, denoted herein as firstarrows 511. The downward force exerted in the first direction maycompress fifth ring 508 into each of the fourth ring 506, third ring505, the second ring 504, and the first ring 502 when the downward forceexceeds the preload of the hoop spring of FIG. 5A. Further still, thethird ring 505 may also be pressed into first ring 502. The compressionof the hoop spring of FIG. 5A occurs due to fifth ring 508 sliding onthe plurality of rolling elements 507 in fourth ring 506 in the firstdirection denoted by first arrows 511. Compression of the hoop spring ofFIG. 5A may be further enabled by the third ring 505 sliding past theplurality of rolling elements in second ring 504 in the first directionas denoted by first arrows 511. As such, first arrows 511 indicate adownward motion. To elaborate, the downward direction may be withrespect to the piston crown sliding towards the piston trunk of piston36 of FIG. 2.

Second image 520 depicts the example hoop spring of FIG. 5A in anexpanded state. When the downward force (e.g., cylinder pressure) doesnot exceed the preload of the hoop spring, fifth ring 508 may bereleased from each of the fourth ring 506, third ring 505, the secondring 504, and the first ring 502 in the second direction, denoted hereinas second arrows 521. Specifically, cylinder pressure may be lower thanthe preload on the hoop spring of FIG. 5A enabling an expansion of thefive rings of the hoop spring of FIG. 5A. The expansion of the hoopspring of FIG. 5A occurs due to fifth ring 508 sliding on the pluralityof rolling elements 507 in fourth ring 506 in the second directiondenoted by second arrows 521. Expansion of the hoop spring of FIG. 5Amay be further enabled by the third ring 505 sliding past the pluralityof rolling elements in second ring 504 in the second direction asdenoted by second arrows 521.

In FIG. 5B, a distinct embodiment of another example hoop springcomprising a first set of springs 532 and a second set of springs 534 isshown. Herein, each of the first set of springs 532 and the second setof springs 534 may comprise three rings arranged concentrically. Thefirst set of springs 532 includes first ring 531, second ring 533, andthird ring 535. Further, the second set of springs 534 includes firstring 536, second ring 537, and third ring 538.

In this embodiment of FIG. 5B, the second set of springs 534 may beconcentrically arranged within the first set of springs 532 relative tocentral axis 501. Thus, first ring 531 of the first set of springs 532may be larger (e.g., have a greater diameter) than the first ring 536 ofthe second set of springs 534. Similarly, second ring 533 of the firstset of springs 532 may be larger (e.g., have a larger diameter) than thesecond ring 537 of the second set of springs 534. Likewise, third ring535 of the first set of springs 532 may be larger than the third ring538 of the second set of springs 534. In other words, the first set ofsprings 532 may be arranged to enclose the second set of springs 534. Assuch, each of the first set of springs 532 and second set of springs 534may be arranged similarly to hoop spring 220 of FIGS. 2, 3, and 4. Thus,each of the second rings (e.g., 533 and 537, respectively) of the firstset of springs 532 and second set of springs 534 may include a pluralityof apertures with each aperture 509 retaining a rolling element 507within. Furthermore, each of the first rings and the third rings of thefirst set of springs and the second set of springs may not includeapertures on their respective circumferences, and therefore, may notfeature any rolling elements. Further still, each of the first set ofsprings 532 and second set of springs 534 may function similarly to hoopspring 220 of FIGS. 2, 3, and 4.

First image 530 of FIG. 5B depicts the hoop spring of FIG. 5B in acompressed state. Herein, a downward force may be exerted on the hoopspring of FIG. 5B in the first direction (as shown by first arrows 511).When the downward force (e.g., cylinder pressure) is greater than thepreload of the hoop spring of FIG. 5B, third ring 538 may be compressedinto each of second ring 537 and first ring 536 of the second set ofsprings 534. Thus, the third ring 538 may slide on the plurality ofrolling elements of second ring 537. Simultaneously, the downward forcemay press third ring 535 into each of second ring 533 and first ring 531of the first set of springs 532.

Second image 540 depicts the example hoop spring of FIG. 5B in anexpanded state. Herein, the downward force (e.g., cylinder pressure)does not exceed the preload of the hoop spring of FIG. 5B. Accordingly,third ring 538 may be released from each of the second ring 537 and thefirst ring 536 the second set of springs 534. At the same time, thirdring 535 may be released from each of the second ring 533 and the firstring 531 the first set of springs 534 in the second direction (e.g.,second arrows 521).

In the described example above, the preload of the first set of springs532 and the second set of springs 534 may be substantially the same. Inanother embodiment, the preload of the first set of springs 532 may begreater than that of the second set of springs 534. In the cases wherethe preload of the first set of springs 532 is different and dissimilarfrom that of the second set of springs 534, each of the rings of thefirst set of springs and the second set of springs may move distinctly.For example, the preload of the first set of springs 532 may be higherthan that of the second set of springs 534. Herein, the third ring 538may be compressed into each of the second ring 537 and the first ring536 of the second set of springs 534 when the cylinder pressure exceedsthe preload of the second set of springs 534 but does not exceed thepreload of the first set of springs 532. At the same time, the firstring, second ring, and third ring of the first set of springs 532 mayremain in their default expanded or released position. The third ring535 of first set of springs 532 may be compressed into each of thesecond ring 533 and first ring 531 of the first set of springs 532 whencylinder pressure exceeds the preload of the first set of springs.Similarly, the rings of the first set of springs 532 and second set ofsprings 534 may release from each other based on cylinder pressure beinglower than each of their respective preloads.

Now turning to FIG. 5C, another example hoop spring is shown, whereinthe hoop spring of FIG. 5C comprises two cylindrical elements (or rings)arranged concentrically relative to central axis 501. Thus, the hoopspring of FIG. 5C includes first ring 552 and third ring 556. A secondring, as in the hoop spring 220 of FIG. 3, may not be included in thisembodiment. Specifically, in the depicted embodiment, the second ring224 of hoop spring 220 having the plurality of rolling elements 230 maynot be provided. Thus, the hoop spring of FIG. 5C does not include anintermediate ring with a plurality of apertures on its circumference. Inother words, the hoop spring of FIG. 5C does not include rollingelements for enabling compression and expansion of the hoop spring. Toelaborate, neither first ring 552 nor third ring 556 include a pluralityof apertures nor rolling elements on their respective circumferences.Further still, the first ring 552 may substantially encircle third ring556.

As shown in FIG. 5C, the first ring 552 and third ring 556 may be directcontact with each other. However, a low friction coating 557 (indicatedby dots) may be applied to an outer lateral surface 558 of the thirdring 556 and/or an inner lateral surface 554 of the first ring 552. Thelow friction coating 557 may allow desirable hysteresis (e.g., frictionand/or damping) of the hoop spring. To elaborate, the low frictioncoating 557 may be applied to an external circumference of third ring556. At the same time, the low friction coating may also be applied toan internal circumference of the first ring 552. In other words,surfaces of the first ring and the third ring that are in direct contactwith each other may be coated with the low friction coating. The lowfriction coating 557 may enable sliding between the third ring 556 andfirst ring 552. Specifically, the third ring 556 may slide into(compress) or out of (release from) the first ring 552.

In alternative embodiments (not shown), the low friction coating 557 maybe applied to each of the plurality of rolling elements 230 in secondring 224 of hoop spring 220. As such, application of the low frictioncoating may facilitate sliding of the third ring 226 against the secondring 224, and the second ring 224 against the first ring 222.

In yet another embodiment (not shown), low friction coating 557 may beapplied to each of the inner lateral surface 232 of the first ring 222and the outer lateral surface 246 of the third ring 226. Herein, thehoop spring may include second ring 224 with a plurality of rollingelements 230. Further, the plurality of rolling elements 230 may not becoated with the low friction coating. As such, application of the lowfriction coating 557 may facilitate sliding of each of the plurality ofrolling elements 230 of the second ring 224 against each of the thirdring 226 and the first ring 222.

First image 550 of FIG. 5C shows the hoop spring of FIG. 5C in acompressed state. Herein, a downward force, such as from a crown of apiston shifting towards a trunk of the piston when cylinder pressureexceeds a preload of hoop spring of FIG. 5C, may be exerted on the hoopspring of FIG. 5C in the first direction (e.g., direction denoted byfirst arrows 511). The downward force may compress third ring 556 intothe first ring 552 when the downward force exceeds the preload of thehoop spring of FIG. 5C. Specifically, outer lateral surface 558 of thirdring 556 may slide over inner lateral surface 554 of first ring 552. Inother words, exterior circumference of third ring 556 may glide alonginternal circumference of first ring 552 such that the first ring 552encompasses third ring 556 more tightly and completely than when in therelaxed state.

Second view 560 depicts the example hoop spring of FIG. 5C in anexpanded state. The expanded state may result when the downward force islower than the preload of the hoop spring of FIG. 5C. Herein, third ring556 may be released from the first ring 552 in the second direction(e.g., second direction denoted by second arrows 521). To elaborate,outer lateral surface 558 of third ring 556 may slide along innerlateral surface 554 of first ring 552 in the second direction. Thesliding enables a release of the third ring from the first ring suchthat the first ring 552 may not encompass the third ring 556 as tightlyas in the compressed state.

In yet another embodiment shown in first image 570 of FIG. 5D, the hoopspring includes three rings similar to hoop spring 220 of FIGS. 2, 3,and 4. Hoop spring of FIG. 5D includes first ring 522, second ring 524,and third ring 526. A circumference of second ring 524 includes multipleapertures 528 with a rolling element 507 positioned within each of themultiple apertures. Similar to hoop spring 220, the hoop spring of FIG.5D functions with third ring 526 sliding over the plurality of rollingelements 507 of the second ring 524. Further, second ring 524 may alsoslide into first ring 522 due to the rolling elements. However, each ofthe first ring 522 and the third ring 526 of the hoop spring of FIG. 5Dmay be formed with variable widths. For example, a width of an upper endW1 of the first ring 522 may be less than a width of a lower end W2 ofthe first ring 222. As an example, the upper end of first ring 522 mayindicate an end (or edge) of first ring 522 that is adjacent to roofsurface 262 of crown portion 202 of piston 36 in FIG. 2. Herein, thelower end of first ring 522 may represent an end (or edge) of first ring522 that is resting on upper rim 258 of trunk portion 250 of piston 36.Thus, the upper end of the first ring 522 may be positioned opposite thelower end of first ring 522.

Similarly, a width of an upper end W3 of the third ring 526 may begreater than a width of a lower end W4 of the third ring 226. Herein,the upper end of third ring 526 may indicate an end (or edge) of thirdring 526 that is adjacent to roof surface 262 of crown portion 202 ofpiston 36 in FIG. 2. Further, the lower end of third ring 526 mayrepresent an end (or edge) of third ring 526 that is resting on upperrim 258 of trunk portion 250 of piston 36. Thus, the upper end of thethird ring 526 may be positioned opposite the lower end of third ring526.

In one example, the width of the upper end W1 of the first ring 222 maybe 1 mm less than the width of the lower end W2 of the first ring 522.Further, the width of the upper end W3 of the third ring 526 may begreater than the width of the lower end W4 of the third ring 526 byabout 1 mm. In yet another example, the width of the upper end W1 of thefirst ring 522 may be smaller than the width of the lower end W2 of thefirst ring 522 by 3 mm. Similarly, the width of the upper end W3 of thethird ring 526 may be 3 mm greater than the width of the lower end W4 ofthe third ring 526. In some examples, a width of the second ring 524 mayalso be varied. In this way, a cross-section of each ring of the hoopspring of FIG. 5D may be varied.

First image 570 of FIG. 5D shows the hoop spring of FIG. 5D in acompressed state wherein a downward force (due to higher cylinderpressure) may be exerted on the hoop spring in the first direction(e.g., first arrows 511). The downward force in the first direction maycompress third ring 526 into each of the second ring 524 and the firstring 522 when the downward force (e.g., cylinder pressure) exceeds thepreload of the hoop spring of FIG. 5D. The third ring 526 may slide overthe plurality of rolling elements 507 of second ring 524 as the hoopspring is compressed. Since the widths of each ring of the hoop springof FIG. 5D are different, the compression capability of this hoop springmay be considerably different from that of hoop spring 220 of FIG. 3.For example, hoop spring of FIG. 5D may be stiffer relative to hoopspring 200 of FIGS. 2 and 3.

Second image 580 of FIG. 5d depicts the example hoop spring in anexpanded state. Second image 580 illustrates a situation when thedownward force is lower than the preload of the hoop spring of FIG. 5D.For example, pressure in the cylinder may be lower than the preload ofhoop spring of FIG. 5D. Herein, third ring 526 may be released from thefirst ring 522 in the second direction (e.g., second arrows 521). Assuch, third ring 526 may shift away from first ring 522 when thedownward force is lower than the preload of the hoop spring enabling ashift of the crown portion of the piston away from the trunk portion ofthe piston.

In this way, different embodiments of the hoop spring may becontemplated. In one example, a number of rings of the hoop spring(e.g., as shown in FIG. 5A) may be modified. For example, the number ofrings in the hoop spring may be increased or decreased. In anotherexample, the widths of each of the rings of the hoop spring (as shown inFIG. 5D0 may be changed. In yet another example, additives, such as lowfriction coatings, may be applied on face-sharing contact surfaces ofthe rings and rolling elements may not be utilized. In still anotherexample, the rings of the hoop spring may be arranged in a nested manneras shown in FIG. 5A. In a further example, multiple sets of the hoopspring (such as hoop spring 220) may be arranged concentrically. Theaforementioned modifications to the hoop spring may achieve a desiredspring rate and/or preload. Further, a profile (linear, progressive,etc.) and cross-section (constant, increasing/decreasing, etc.) of thehoop spring may also be adjusted depending on desired springcharacteristics. As a result, engine efficiency may be improved. Turningnow to FIG. 6, a sectional view of an example piston, such as piston 36,is shown schematically. The piston comprises an upper crown portion(e.g., crown portion 202), a lower trunk portion, (e.g., trunk portion250), and a spring (e.g., hoop spring 220) mounted therebetween atop thetrunk portion and within the crown portion. Hoop spring 220 is shown ina first state, or compressed position in FIG. 6A, while FIG. 6B depictshoop spring 220 in a second state, or expanded position. The position ofhoop spring 220 may be related to a position of the crown portion 202relative to the position of the trunk portion 250 of piston 36. Duringan engine cycle, hoop spring 220 in piston 36 may be in the first state,the second state, and/or any intermediate state therebetween, dependingon one or more engine operating conditions.

As shown in FIG. 6A, hoop spring 220 is in the compressed position.Further, piston 36 is also in the compressed position such that lowerrim 264 of crown portion 202 abuts against the upper rim 258 of trunkportion 250. As described earlier, the hoop spring may be compressedwhen a pressure in the cylinder is higher than a preload on the hoopspring. To elaborate, as cylinder pressure exceeds the preload of thehoop spring, the crown portion of the piston may be displaced towardsthe trunk portion of the piston causing compression of the hoop spring.

FIG. 6A shows a gap between lower rim 264 of crown portion 202 and upperrim of trunk portion 250 for clarifying the contact between the tworims. However, in actuality, the lower rim 264 of crown portion 202 andupper rim of trunk portion 250 may be substantially in direct contactwith each other. Accordingly, a distance D between the lower rim 264 ofcrown portion 202 and the upper rim 258 of trunk portion 250 may besubstantially zero. When the hoop spring 220 within the crown portion202 is compressed in the compressed position of the piston, each of thesecond ring 224 and the third ring 226 of hoop spring 220 may be pressedinto the first ring 222 of hoop spring 220.

In the depicted example of FIG. 6A, roof surface 262 of crown portion202 may exert a downward force on hoop spring 220 in the first directiontowards the trunk portion 250, denoted herein as first arrows 511.Specifically, the third annular element (or third ring 226) of hoopspring 220 may be pressed into each of the second annular element (orsecond ring 224) and the first annular element (or first ring 222).Further, the third annular element may move past the plurality ofrolling elements in the second annular element in the first directionsuch that the third annular element may be circumferentially enclosed bythe second annular element. The described position of the hoop springmay occur during one or more strokes of the piston, such as a powerstroke when cylinder pressure is higher than the preload on the hoopspring.

In one embodiment, the crown portion 202 may exert the downward force inthe first direction during one or more operating conditions (e.g.,higher engine loads), and one or more strokes of the cylinder cycle. Forexample, when an engine load exceeds a load threshold, cylinder pressurewithin the combustion chamber may increase. As a result, the increasedcylinder pressure may apply a force on the crown portion 202 exceedingthe preload of the hoop spring 220, as discussed in reference to FIG. 2.Accordingly, hoop spring 220 may be compressed and compacted together asshown in FIG. 6A.

The compressed position of piston 36 may allow an increased volume inthe combustion chamber, such as cylinder 30 of FIG. 1, during anexpansion stroke in the cylinder cycle, for example. As such, theincreased volume in cylinder 30 may lead to a lower compression ratio.In one example, the lower compression ratio may be approximately 9.5:1.In this way, risk of knock and excessive thermal load caused by anincreased cylinder pressure during higher engine load conditions may belessened. As such, arranging the hoop spring 220 within the crownportion 202 may reduce impact shock between lower rim 264 of the crownportion 202 and upper rim 258 of the trunk portion 250 when distance Dis decreasing. This reduction of impact shock is due to the hoop spring220 having a specific spring rate and preload. Specifically, the springrate and preload of hoop spring 220 may be selected such that crownportion 202 will move to the compressed position when a cylinderpressure exceeds a predetermined threshold e.g. preload of hoop spring220.

FIG. 6B shows the example piston in an extended, or released, state withhoop spring 220 at an expanded position. It will be noted that thereleased position of the hoop spring (and piston) may be assumed whencylinder pressure is lower than the preload on the hoop spring 220.Herein, lower rim 264 of crown portion 202 does not contact the upperrim 258 of trunk portion 250. In other words, a base of crown portion202 is separated from trunk portion 250 by distance D1. As such,distance D1 may be greater than distance D in FIG. 6A. In one example,distance D1 between the lower rim 264 of crown portion 202 and the upperrim 258 of trunk portion 250 may be 3 mm. In another example, thedistance D1 may be greater than 4 mm. In another example, the distanceD1 may be greater than 5 mm. In yet another example, distance D1 may bea distance between 3 and 4 mm.

When piston 36 is in the expanded position, hoop spring 220 within thecrown portion 202 may not be in the compressed state, as described inreference to FIG. 2. For example, when cylinder pressure reduces belowthe preload of the hoop spring, the rings of the hoop spring may releasefrom each other (e.g., spread apart) allowing the hoop spring to assumean expanded position as shown in FIG. 6B. In the expanded position ofthe hoop spring 220, the third ring 226 may be released from each of thesecond ring 224 and the first ring 222. Further, the second ring 224moves in the second direction, and the third ring 226 moves in the same,second direction. As stated above, the second direction may be towardscrown portion 202 of piston 36, denoted here as second arrows 521. Assuch, the second direction may be the opposite of the first direction.

In one embodiment, the hoop spring 220 may exert an upward force on thecrown portion 202 in the second direction during one or more operatingconditions, and one or more strokes of the cylinder cycle. For example,when an engine load is lower, cylinder pressure within the combustionchamber may reduce. As a result, the spring assembly may release fromits compressed state and exert a force on the crown portion 202 awayfrom trunk portion 250 of piston 36, as discussed in reference to FIG.2. Accordingly, hoop spring 220 may be expanded, such that the thirdring 226 is at least partially released from second ring 224, and secondring 224 is at least partially released from the first ring 222, asshown in FIG. 6B.

In sum, the expanded position may enable a higher compression ratio ascompared to when the piston is in compressed position (FIG. 6A). In oneexample, the higher compression ratio may be approximately 13.5:1. Asshown in FIG. 6B, crown portion 202 is positioned at distance D1 fromtrunk portion 250 as compared to the piston in the compressed position.As such, hoop spring 220 is in a relatively extended position since theroof surface 262 of crown portion 202 may no longer be applying adownward force on the upper rim 238 of the third ring 226 greater thanthe preload of the spring assembly.

As such, the expanded position of piston 36 may allow a decreased volumein the combustion chamber, such as cylinder 30 of FIG. 1. The decreasedvolume in cylinder 30 may lead to a higher compression ratio. In thisway, engine efficiency may be increased, thereby increasing fueleconomy.

Accordingly, FIGS. 6A and 6B illustrate adjustments to the position ofthe crown of the piston relative to the position of the trunk portion ofthe piston. In one example, the adjustments may include decreasing adistance (e.g., distance D1) between the crown of the piston and thetrunk of the piston by compressing the third annular element (e.g.,third ring 226) into each of the second annular element (e.g., secondring 224) and the first annular element (e.g., first ring 222), thethird annular element moving past the plurality of rolling elements(e.g., rolling elements 230) in the second annular element in a firstdirection such that the third annular element may be circumferentiallyenclosed (e.g., substantially) by the second annular element.

In another example, the adjustments may include increasing a distancebetween the crown of the piston and the trunk of the piston by releasingthe third annular element from each of the second annular element andthe first annular element, wherein the third annular element moves pastthe plurality of rolling elements in the second annular element in asecond direction during a second stroke of the piston, the seconddirection opposite to the first direction.

Thus, systems and methods are provided, comprising varying a compressionratio of an engine via a spring assembly located in a crown of a pistonin a cylinder of the engine, the spring assembly comprising a firstannular element fitted inside a second annular element, and a thirdannular element enclosing the second annular element, wherein the secondannular element includes a plurality of apertures with a rolling elementpositioned within each of the plurality of apertures. In one embodiment,the spring assembly may have a preload. Further, as shown in FIGS. 2-5,a central axis of the first annular element may be parallel to each of acentral axis of the second annular element and a central axis of thethird annular element.

In one embodiment, varying the compression ratio of the engine mayinclude adjusting a position of the crown of the piston relative to aposition of a trunk of the piston. Further, adjusting the position ofthe crown of the piston relative to the position of the trunk of thepiston may include decreasing a distance between the crown of the pistonand the trunk of the piston. More specifically, in one example,decreasing the distance between the crown of the piston and the trunk ofthe piston may include compressing the first annular element into eachof the second annular element and the third annular element, the firstannular element moving past the plurality of rolling elements in thesecond annular element such that the first annular element may becircumferentially enclosed by the second annular element.

Further, adjusting the position of the crown of the piston relative tothe position of the trunk of the piston may also include increasing thedistance between the crown of the piston and the trunk of the piston. Inthis example, increasing the distance between the crown of the pistonand the trunk of the piston may include releasing the first annularelement from each of the second annular element and the third annularelement, wherein the first annular element moves past the plurality ofrolling elements in the second annular element in a direction away fromeach of the second annular element and the third annular element.

FIG. 7 shows an example routine 700 for adjusting spark timingresponsive to an estimated compression ratio in an engine, such asengine 10 of FIG. 1. Specifically, a first setting for spark timing maybe determined based on existing engine conditions. The first setting maybe adjusted further based on the estimated compression ratio in theengine.

Instructions for carrying out routine 700 may be executed by acontroller, such as controller 12 of FIG. 1, based on instructionsstored on a memory of the controller and in conjunction with signalsreceived from sensors of the engine system, such as the sensorsdescribed above with reference to FIG. 1. The controller may employengine actuators of the engine system to adjust engine operationaccording to the routine described below.

At 702, routine 700 estimates and/or measures engine operatingconditions. Engine operating conditions may include engine speed, engineload, engine temperature, age of the engine, etc. The age of the enginemay be determined based on a mileage of the vehicle since enginemanufacture. Alternatively, age of the engine may be based on a numberof combustion events. In addition, at 702, routine 700 estimates ambientconditions such as ambient temperature and humidity, barometricpressure, etc.

Next, at 704, routine 700 determines a first setting for spark timingbased on engine operating conditions measured and/or estimated at 702.In particular, the first setting for spark timing may be a function ofthe measured engine speed and estimated engine load. Additionalparameters such as exhaust gas recirculation (EGR), altitude, airtemperature, etc. may also determine the first setting for spark timing.In one example, engine load may be estimated by a sensor (not shown)which measures an amount of vacuum produced by the engine. In anotherexample, the engine speed may be measured by a crankshaft positionsensor, such as Hall Effect sensor 118 of FIG. 1, detecting an angularrotational speed of the engine crankshaft in revolutions per minute.

At 706, routine 700 estimates a change in compression ratio of theengine. At 707, compression ratio of the engine may depend upon enginespeed, engine torque, and cylinder pressure, as well as compression ofthe hoop springs within each piston of the engine. Herein, compressionof the hoop springs may be affected by wear and degradation of thecomponents of the hoop spring. Accordingly, routine 700 estimates theage of the engine to determine functionality and robustness of the hoopspring. As mentioned earlier, the age of the engine may be determinedbased on engine operation since a date of engine manufacture. In anotherexample, the age of the engine may also be learned by vehicle mileage.In particular, the age of the engine may help infer performance of thehoop spring, and thus an estimation of preload and elasticity of thehoop springs in each piston. Over time, the preload and/or spring rateof the hoop spring may change. In one example, the preload and/or springrate of the hoop spring may decrease as the age of the engine increases.Therefore, an ability of the hoop spring to adjust engine compressionratio may decline.

After the change in compression ratio is determined at 706, routine 700adjusts the first setting for spark timing at 708 based on the estimatedchange in compression ratio. Spark timing may be adjusted (e.g.,advanced or retarded) based on compression ratio to maintain or increaseengine efficiency and performance, while reducing a risk of knock. Inone example, spark timing may be adjusted uniformly for all cylinders.In another example, spark timing may be adjusted to providecylinder-by-cylinder control of spark timing in order to equalizeburn-rates or to retard the spark for knock-prone cylinders.

After any adjustment(s) to spark-timing are completed by the controller,routine 600 ends. In this way, engine efficiency and engine performancemay be increased by adjusting spark timing based on the variablecompression ratio of the engine and specifically, the performance of thehoop spring.

Thus, a method may be provided, comprising operating an engine with afirst setting for spark timing, the engine including a cylinder, and thecylinder including a piston with a spring positioned in a crown of thepiston, and adjusting the first setting for spark timing responsive toan estimated compression ratio of the engine, the compression ratiobased on an age of the spring. In one example, the age of the spring maybe based on an age of the engine, the age of the engine based on anumber of engine cycles. In another example, the compression ratio maybe further based on one or more of an engine speed and a pressure in thecylinder of the engine.

In another example, the method may also comprise adjusting a sparktiming based on the varying compression ratio of the engine, the varyingof the compression ratio based upon an engine speed, a pressure in eachcylinder of the engine, and an age of the engine. Accordingly, the ageof the engine determines a robustness of the spring assembly.

Further, the spring may include a first ring, a second ring, and a thirdring arranged in a concentric manner. Specifically, the third ring maybe substantially surrounded by the second ring, and the second ring maybe substantially surrounded by the first ring. Further, thecircumference of the second ring may comprise a plurality of apertures,while a circumference of the first ring and a circumference of the thirdring may not include a plurality of apertures. In the second ring, arolling element may be retained in each of the plurality of apertures.

In one embodiment, the spring may have a preload. During someconditions, the spring may be compressed in response to the pressure inthe cylinder being higher than the preload. In this example, the thirdring may be pressed into each of the second ring and the first ring, thethird ring sliding on rolling elements positioned in the plurality ofapertures in the second ring as a distance between the crown of thepiston and a piston trunk decreases.

During other conditions, the third ring 226 may release from each of thesecond ring 224 and the first ring 222 in response to the pressure inthe cylinder being lower than the preload, the third ring 226 sliding onthe rolling elements positioned in the plurality of apertures in thesecond ring. In this example, the release of the third ring and secondring from the first ring may increase a distance between the pistoncrown and the piston trunk.

The technical effect of implementation of a hoop spring comprising afirst ring, a second ring, and a third ring within a piston crown may bean improvement to engine efficiency and fuel economy. The improvement toengine efficiency and fuel economy is due, in part, by beneficial loaddeflection, or spring rate, characteristics of the hoop spring, a moreeven stress distribution, as well as a reduction in piston weight andpackage size of the engine due to the hoop spring having less mass andsize as compared to the Belleville washers. Further, the more evenstress distribution of the hoop spring may increase fatigue life due toa higher durability as compared to the Belleville springs. As a result,the higher engine efficiency resulting from a higher compression ratioduring low loads may be combined with the knock control available with alower compression ratio in a PRP system. Thus, the use of expensive,higher-octane fuels may be avoided.

In another representation, a system may be provided, comprising, anengine including a cylinder, a piston reciprocating within the cylinder,the piston including a piston crown and a piston trunk, a connectingrod, a first end of the connecting rod coupled to the piston trunk via awrist pin, a second end of the connecting rod coupled to a crank pin ofa crankshaft of the engine, and a spring situated on a top surface ofthe piston trunk and surrounded by the piston crown, the spring formedof a first ring, a second ring, and a third ring arranged in aconcentric manner, wherein the third ring may be nested within thesecond ring, and the second ring may be nested within the first ring, acircumference of the second ring including multiple apertures, and acircumference of the first ring and a circumference of the third ringnot including multiple apertures.

Further, in one example, each of the multiple apertures on thecircumference of the second ring may include a rolling element, and eachof the first ring, the second ring, and the third ring may be tapered.In addition, the second ring may be formed of a polymer material whileeach of the first ring and the third ring may be formed of a metal.

It will be appreciated that the configurations and routines disclosedherein are exemplary in nature, and that these specific embodiments arenot to be considered in a limiting sense, because numerous variationsare possible. For example, the above technology can be applied to V-6,I-4, I-6, V-12, opposed 4, and other engine types. The subject matter ofthe present disclosure includes all novel and non-obvious combinationsand sub-combinations of the various systems and configurations, andother features, functions, and/or properties disclosed herein.

The following claims particularly point out certain combinations andsub-combinations regarded as novel and non-obvious. These claims mayrefer to “an” element or “a first” element or the equivalent thereof.Such claims should be understood to include incorporation of one or moresuch elements, neither requiring nor excluding two or more suchelements. Other combinations and sub-combinations of the disclosedfeatures, functions, elements, and/or properties may be claimed throughamendment of the present claims or through presentation of new claims inthis or a related application. Such claims, whether broader, narrower,equal, or different in scope to the original claims, also are regardedas included within the subject matter of the present disclosure.

1. A system, comprising: a piston crown; and a spring positioned withinthe piston crown, the spring including a first ring, a second ringcomprising a plurality of apertures, a rolling element positioned withineach of the plurality of apertures, and a third ring, wherein the firstring, the second ring, and the third ring are arranged concentricallywith the second ring positioned between the first ring and the thirdring.
 2. The system of claim 1, wherein the first ring is in directcontact with the second ring, and not in direct contact with the thirdring, and wherein the third ring is in direct contact with the secondring but not in direct contact with the first ring.
 3. The system ofclaim 2, wherein a central axis of the first ring is parallel to each ofa central axis of the second ring and a central axis of the third ring.4. The system of claim 3, wherein each of the first ring, the secondring, and the third ring are shaped as tapered cylinders.
 5. The systemof claim 4, wherein each of the first ring and the third ring are madeof steel, and the second ring is made of a polymer.
 6. The system ofclaim 5, wherein a central axis of the piston crown coincides with eachof the central axis of the first ring, the central axis of the secondring, and the central axis of the third ring.
 7. The system of claim 6,further comprising a piston trunk coupled to a base of the piston crown,and wherein the spring is positioned on a top of the piston trunk. 8.The system of claim 7, wherein the piston crown is movable relative tothe piston trunk, and wherein the piston trunk is fixed relative to thepiston crown.
 9. A method, comprising: varying a compression ratio of anengine via a spring assembly located in a crown of a piston in acylinder of the engine, the spring assembly comprising a third annularelement fitted inside a second annular element, and a first annularelement enclosing the second annular element, wherein the second annularelement includes a plurality of apertures with a rolling elementpositioned within each of the plurality of apertures.
 10. The method ofclaim 9, wherein a central axis of the first annular element is parallelto each of a central axis of the second annular element and a centralaxis of the third annular element.
 11. The method of claim 10, whereinvarying the compression ratio of the engine includes adjusting aposition of the crown of the piston relative to a position of a trunk ofthe piston, and wherein the spring assembly has a preload.
 12. Themethod of claim 11, wherein adjusting the position of the crown of thepiston relative to the position of the trunk of the piston includesdecreasing a distance between the crown of the piston and the trunk ofthe piston, the decreasing including compressing the third annularelement into each of the second annular element and the first annularelement, the third annular element moving past the plurality of rollingelements in the second annular element such that the third annularelement is circumferentially enclosed by the second annular element. 13.The method of claim 12, wherein adjusting the position of the crown ofthe piston relative to the position of the trunk of the piston includesincreasing the distance between the crown of the piston and the trunk ofthe piston, the increasing including releasing the third annular elementfrom each of the second annular element and the first annular element,wherein the third annular element moves past the plurality of rollingelements in the second annular element in a direction away from each ofthe second annular element and the first annular element.
 14. The methodof claim 9, further comprising adjusting a spark timing based on thevarying compression ratio of the engine, the varying compression ratiobased upon an engine speed, a pressure in each cylinder of the engine,and an age of the engine, and wherein the age of the engine determines arobustness of the spring assembly. 15-20. (canceled)